Off-grid power generating apparatus and frequency and voltage control method thereof

ABSTRACT

A power generating apparatus is provided. The alternator includes a rotor, a stator, one or more sensors and an electrical circuit. The rotor includes a plurality of symmetric phase windings while the stator has a single phase winding. The excitation control device is configured to control the induced voltage generated in stator by regulating the rotating magnetic field generated in the phase windings of the rotor. The excitation control device is also configured to regulate the engine speed responsive to calculated load power. The electrical circuit connecting the single phase winding of the stator and the load is configured in a way that the induced voltage generated in the single phase winding and the output voltage applied to the load are at the same frequency. This arrangement reduces costs of the apparatus.

RELATED APPLICATION

This application claims priority to Chinese Patent Application No.201810073126.X, entitled “Off-Grid Power Generating Apparatus andFrequency and Voltage Control Method Thereof” filed on Jan. 25, 2018,which is incorporated by reference in its entirety.

TECHNICAL FIELD

Embodiments disclosed herein relate generally to an electrical powergenerating apparatus and its control method, and in particular, to anengine-driven and off-grid power generating apparatus and its controlmethod with variable speed and constant frequency.

BACKGROUND

An off-grid generator set is a power generating system whose connectionto an electrical power grid is temporarily or permanently unavailable.Off-grid generator sets have been widely used. For example, aportable/standby generator set may be utilized to power electricalequipment connected via a circuit with the generator set when people docamping, tailgating and working outside. A similar power generatingapparatus is also utilized to provide backup power in emergencies, forexample during blackout.

The off-grid generator set usually utilizes a driving engine coupled toa generator (or alternator) through a common shaft. Upon actuation ofthe engine, the engine rotates the common shaft to drive the alternatorthat, in turn, generates electrical power. As it is known, mostresidential electric equipment is designed to be used in connection withelectrical power having a fixed frequency, for example, 60 hertz (Hz) inNorth America and 50 Hz in China. The frequency of the output electricalpower is primarily determined by the operating speed of the engine. As aresult, the engine's operating speed of some generator sets is fixed asso to keep the frequency of the output electrical power fixed. However,in situations where an electrical load applied to the generator set isless than the rated kilowatt load for which the generator set isdesigned, the engine's fuel-efficiency deteriorates. The generator setgenerates loud noise.

Additionally, the off-grid generator set usually generates an outputvoltage at a certain level since most electrical loads are designed towork with a voltage at such a level. For example, most householdappliances in North America such as ovens, roasters, audio and videodisplayers use nominal voltages of 120 volts or 240 volts, and mosthousehold appliances in China use a nominal voltage of 220 volts.

SUMMARY

Embodiments of off-grid power generating apparatus and its voltage andfrequency control method are described. The off-grid power generatingapparatus includes a stator with one single phase winding and a rotorwith a plurality of symmetric phase windings. This feature of theapparatus helps to make it lighter and cheaper. This apparatus, however,poses a challenge to conventional control strategies for generators. Theinventors have contemplated to apply vector control strategies to suchan apparatus with a single phase winding on the stator side. However,the vector control strategies are usually used to control three phasegenerators. Complex functions such as Park-Clark transformations areneeded to transform the single phase parameters of the apparatus intothree phase ones. Moreover, the vector control strategies are themselvescomplex, and expensive meters, particularly an angular position sensorsuch as an alternator encoder, are needed for implementing this type ofstrategy. A simple and cost-effective control system for the apparatusis provided in this disclosure. The control system modulates the speedof the driving engine in response to load changes for minimizing fuelconsumption. Moreover, the amplitude and frequency of the excitationcurrent in the rotor windings are modulated to make the amplitude andfrequency of the output voltage from the stator constant (i.e., toachieve constant voltage and constant frequency). In this disclosure,the off-grid power generating apparatus is a power generating systemwhose connection to an electrical power grid is temporarily orpermanently unavailable. A single phase winding of the stator refers tothat the coil of the stator has only one axial direction. The singlephase winding of the stator may include a plurality of coils but theaxial direction of the plurality of coils should coincide. Symmetricphase windings are windings configured to form a rotating magnetic fieldthat is an air gap magnetic field of a motor with a constant magnitude.The plurality of symmetric phase windings may have two or more phases.

The off-grid power generating apparatus for powering an electrical loadincludes an engine, an alternator and an excitation control device inaccordance with some embodiments. The alternator includes a rotor, astator, a voltage sensor, a current sensor and a speed sensor. The rotoris coaxially coupled to the engine, and includes a plurality ofsymmetric phase windings. The stator includes a single phase windingconfigured to generate an induced voltage. The single phase winding isconfigured to be connected with the electrical load and the phasewindings of the rotor respectively and provide an output voltage to theelectrical load and an excitation voltage to the phase windings of therotor. The voltage sensor is configured to measure an amplitude of theoutput voltage. The current sensor is configured to measure theamplitude of the alternating current applied to the electrical load. Thespeed sensor is configured to measure the rotation speed of the rotor orthe engine. The excitation control device is operatively connected withthe engine and the alternator that is configured to control the inducedvoltage generated in the single phase winding of the stator byregulating the rotating magnetic field generated in the phase windingsof the rotor. The excitation control device includes a first calculatingelement, a first modulating element, a second calculating element, athird calculating element and a second modulating element. The firstcalculating element is configured to calculate the load power of theelectrical load at least in accordance with the measured amplitudes ofthe output voltage and the alternating current, and determine a desiredrotation speed of the engine at least in accordance with the calculatedload power. The first calculating element may be implemented by twomodules, which separately perform the step of calculating the load powerand the step of obtaining the desired rotation speed. The firstmodulating element is configured to modulate the speed of the engine atleast in accordance with the desired rotation speed of the engine. Thesecond calculating element is configured to determine a slip angle atleast in accordance with the rotation speed of the rotor or the enginemeasured by the speed sensor. The third calculating element isconfigured to determine a target voltage of the rotor at least inaccordance with the amplitude of the output voltage measured by thevoltage sensor. The second modulating element is configured to generatea modulating signal at least in accordance with the target voltage ofthe rotor and the slip angle, and modulate a frequency and an amplitudeof an excitation current in the phase windings of the rotor with themodulating signal.

The alternator comprising a frequency conversion device connected withthe single phase winding of the stator in accordance with someembodiments. The frequency conversion device includes an inverter toprovide the excitation voltage to the phase windings of the rotor.

The alternator includes a switch operatively connected with theelectrical load and being movable from a first position to a secondposition by a user in accordance with some embodiments. The single phasewinding of the stator includes a first segment and a second segment eachof which has at least one coil that are operatively and separatelyconnected with the switch. The output voltage includes a high outputvoltage and a low output voltage. The first segment and the secondsegment are configured to be in series connection at the first positionof the switch and in parallel connection at the second position of theswitch to provide the high output voltage and the low output voltagerespectively to the electrical load via the switch. The voltage sensoris configured to measure the amplitudes of a first and a second outputvoltage provided by the first and the second segment to the electricalload. The current sensor is configured to measure the amplitudes of afirst and a second alternating current applied respectively by the firstand the second segment to the electrical load. The first calculatingelement is configured to calculate a first and a second load power ofthe electrical load at least in accordance with the measured amplitudesof the first output voltage and the first alternating current, and thesecond output voltage and the second alternating current, and calculatea total load power by adding the first and the second load power, andobtain the desired rotation speed of the engine at least in accordancewith the total load power. The third calculating element is configuredto determine a target voltage of the rotor at least in accordance withthe amplitude of the first or the second output voltage.

The alternator includes a switch that is operatively connected with theelectrical load and is movable from a first position to a secondposition by a user. The single phase winding of the stator includes afirst segment and a second segment each of which has at least one coilthat are operatively and separately connected with the switch. Theoutput voltage includes a high output voltage and a low output voltage.The first segment and the second segment are configured to be in seriesconnection at the first position of the switch and in parallelconnection at the second position of the switch to provide the highoutput voltage and the low output voltage respectively to the electricalload via the switch. The voltage sensor is configured to measure theamplitudes of a first and a second output voltage provided by the firstsegment and the second segment to the electrical load, and a totaloutput voltage when the first segment and the second segment are inseries connection. The current sensor is configured to measure theamplitudes of a first and a second alternating current appliedrespectively by the first and the second segment to the electrical load.When the first segment and the second segment are in series connectionat the first position of the switch to provide the high output voltage,the first calculating element is configured to calculate a total loadpower at least in accordance with the measured amplitudes of the totaloutput voltage and either of the first alternating current and secondalternating current. The first calculating element is also configured toobtain the desired rotation speed of the engine at least in accordancewith the total load power. When the first segment and the second segmentare in parallel connection at the second position of the switch toprovide the low output voltage, the first calculating element isconfigured to calculate a first and a second load power of theelectrical load at least in accordance with the measured amplitudes ofthe first output voltage and the first alternating current, and thesecond output voltage and the second alternating current. The firstcalculating element is also configured to calculate a total load powerby adding the first and the second load power, and to obtain the desiredrotation speed of the engine at least in accordance with the total loadpower. The third calculating element is configured, when the firstsegment and the second segment are in series connection at the firstposition of the switch to provide the high output voltage, to determinea target voltage of the rotor at least in accordance with the amplitudeof the total output voltage. The third calculating element is alsoconfigured, when the first segment and the second segment are inparallel connection at the second position of the switch to provide thelow output voltage, to determine a target voltage of the rotor at leastin accordance with the amplitude of the first or the second outputvoltage.

The third calculating element is configured to determine the targetvoltage of the rotor with a closed control loop in accordance with someembodiments.

The alternator includes an electrical circuit connecting the singlephase winding of the stator and the electrical load in accordance withsome embodiments. The electrical circuit is configured in a way that theinduced voltage generated in the single phase winding of the stator andthe output voltage applied to the electrical load are at the samefrequency.

A control method of an off-grid power generating apparatus for poweringan electrical load is provided in accordance with some embodiments. Theapparatus has an engine and an alternator that includes a stator with asingle phase winding configured to generate an induced voltage and arotor with a plurality of symmetric phase windings. The rotor iscoaxially coupled to the engine. The single phase winding of the statoris connected with the electrical load and the phase windings of therotor respectively to provide an output voltage to the electrical loadand an excitation voltage to the phase windings of the rotor. The methodincludes the operations as follows. Measure the amplitude of the outputvoltage provided to the electrical load. Measure the amplitude of thealternating current provided to the electrical load. Measure therotation speed of the rotor or the engine. Calculate the load power ofthe electrical load at least in accordance with the measured amplitudesof the output voltage and the alternating current. Determine the desiredrotation speed of the engine at least in accordance with the calculatedload power. Modulate the speed of the engine at least in accordance withthe desired rotation speed of the engine. Determine a slip angle atleast in accordance with the measured rotation speed of the rotor or theengine and the synchronous speed of the alternator. Determine the targetvoltage of the rotor at least in accordance with the amplitude of themeasured output voltage. Generate a modulating signal at least inaccordance with the target voltage of the rotor and the slip angle.Modulate the frequency and the amplitude of an excitation current in thephase windings of the rotor with the modulating signal.

In accordance with some embodiments of the method, the alternatorincludes a frequency conversion device connected with the single phasewinding of the stator. The frequency conversion device includes aninverter to provide the excitation voltage to the phase windings of therotor.

In accordance with some embodiments of the method, the alternatorincludes a switch that is operatively connected with the electrical loadand is movable between a first position and a second position by a user.The single phase winding of the stator includes a first segment and asecond segment each of which has at least one coil that is operativelyand separately connected with the switch. The output voltage includes ahigh output voltage and a low output voltage. The first segment and thesecond segment are configured to be in series connection at the firstposition of the switch and in parallel connection at the second positionof the switch to provide the high output voltage and the low outputvoltage respectively to the electrical load via the switch. Measuring anamplitude of the output voltage includes measuring the amplitudes of afirst and a second output voltage provided by the first and the secondsegment to the electrical load. Measuring an amplitude of an alternatingcurrent includes measuring the amplitudes of a first and a secondalternating current applied respectively by the first and the secondsegment to the electrical load. Calculating a load power of theelectrical load includes calculating a first and a second load power ofthe electrical load at least in accordance with the measured amplitudesof the first output voltage and the first alternating current, and thesecond output voltage and the second alternating current, and a totalload power by adding the first and the second load power. Determining adesired rotation speed of the engine at least in accordance with thecalculated load power includes determining the desired rotation speed ofthe engine at least in accordance with the total load power. Determininga target voltage of the rotor includes determining the target voltage ofthe rotor at least in accordance with the amplitude of the first or thesecond output voltage.

In accordance with some embodiments of the method, the alternatorincludes a switch that is operatively connected with the electrical loadand is movable from a first position to a second position by a user. Thesingle phase winding of the stator includes a first segment and a secondsegment each of which has at least one coil that are operatively andseparately connected with the switch. The output voltage includes a highoutput voltage and a low output voltage. The first segment and thesecond segment are configured to be in series connection at the firstposition of the switch and in parallel connection at the second positionof the switch to provide the high output voltage and the low outputvoltage respectively to the electrical load via the switch. Measuring anamplitude of the output voltage includes measuring the amplitudes of afirst and a second output voltage provided by the first segment and thesecond segment to the electrical load, and a total output voltage whenthe first segment and the second segment are in series connection.Measuring an amplitude of an alternating current includes measuring theamplitudes of a first and a second alternating current appliedrespectively by the first and the second segment to the electrical load.When the first segment and the second segment are in series connectionat the first position of the switch to provide the high output voltage,calculating a load power of the electrical load includes calculating atotal load power at least in accordance with the measured amplitudes ofthe total output voltage and either of the first alternating current andsecond alternating current. Determining a desired rotation speed of theengine includes determining the desired rotation speed of the engine atleast in accordance with the total load power. Determining a targetvoltage of the rotor includes determining the target voltage of therotor at least in accordance with the amplitude of the measured totaloutput voltage. When the first segment and the second segment are inparallel connection at the second position of the switch to provide thelow output voltage, calculating a load power of the electrical loadincludes calculating a first and a second load power of the electricalload at least in accordance with the measured amplitudes of the firstoutput voltage, the first alternating current, the second output voltageand the second alternating current, and calculating a total load powerby adding the first and the second load power. Determining a desirerotation speed of the engine includes determining the desired rotationspeed of the engine at least in accordance with the total load power.Determining a target voltage of the rotor includes determining thetarget voltage of the rotor at least in accordance with the amplitude ofthe first or the second output voltage.

An off-grid portable generator set for powering an electrical load isprovided in accordance with some embodiments. The generator set includesan engine, an induction alternator and an excitation control device. Thealternator includes a rotor, a stator, a voltage sensor, a currentsensor and a speed sensor. The rotor is coaxially coupled to the engine,and includes a plurality of symmetric phase windings. The statorincludes a single phase winding configured to generate an inducedvoltage. The single phase winding is connected with the electrical loadand the phase windings of the rotor respectively to provide an outputvoltage to the electrical load and an excitation voltage to the phasewindings of the rotor. The voltage sensor is configured to measure anamplitude of the output voltage. The current sensor is configured tomeasure the amplitude of the alternating current applied to theelectrical load. The speed sensor is configured to measure the rotationspeed of the rotor or the engine. The excitation control device isoperatively connected with the engine and the alternator that isconfigured to control the induced voltage generated in the single phasewinding of the stator by regulating the rotating magnetic fieldgenerated in the phase windings of the rotor. The excitation controldevice includes a first calculating element, a first modulating element,a second calculating element, a third calculating element and a secondmodulating element. The first calculating element is configured tocalculate a load power of the electrical load at least in accordancewith the measured amplitudes of the output voltage and the alternatingcurrent, and determine a desired rotation speed of the engine at leastin accordance with the calculated load power. The first modulatingelement is configured to modulate the speed of the engine at least inaccordance with the desired rotation speed of the engine. The secondcalculating element is configured to determine a slip angle at least inaccordance with the rotation speed of the rotor or the engine measuredby the speed sensor. The third calculating element is configured todetermine a target voltage of the rotor at least in accordance with theamplitude of the output voltage measured by the voltage sensor. Thesecond modulating element is configured to generate a modulating signalat least in accordance with the target voltage of the rotor and the slipangle, and modulate a frequency and an amplitude of an excitationcurrent in the phase windings of the rotor with the modulating signal.

The off-grid power generating apparatus has a stator with a single phasewinding and a rotor with a plurality of symmetric windings that generatea rotating magnetic field. The combination of a stator with a singlewinding and a rotor with a plurality of symmetric phase windings enablesthe apparatus to power single phase electrical devices with small ratedpowers such as household appliances while keeping the control system ofthe apparatus simple and easy to implement. The control method modulatesthe engine speed in response to load changes to minimize fuelconsumption. Moreover, the amplitude and frequency of the excitationcurrent in the rotor windings are modulated to keep the output voltagesoutput from the stator side stable.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of an off-grid power generating apparatusin accordance with some embodiments.

FIG. 2A illustrates an arrangement of the rotor in accordance with someembodiments.

FIG. 2B illustrates another arrangement of the rotor in accordance withsome embodiments.

FIG. 2C illustrates another arrangement of the rotor in accordance withsome embodiments.

FIG. 3 is a schematic diagram illustrating the structure of anexcitation control device in accordance with some embodiments.

FIG. 4 is a schematic diagram of another off-grid power generatingapparatus in accordance with some embodiments.

FIG. 5 is a schematic diagram of yet another off-grid power generatingapparatus in accordance with some embodiments.

FIG. 6A illustrates a DC bus voltage regulator in accordance with someembodiments.

FIG. 6B illustrates another DC bus voltage regulator in accordance withsome embodiments.

FIG. 7 illustrates another embodiment of providing an excitation voltageto the phase windings of the rotor in accordance with some embodiments.

FIG. 8 is a schematic diagram of another off-grid power generatingapparatus in accordance with some embodiments.

FIG. 9 is a schematic diagram of yet another off-grid power generatingapparatus in accordance with some embodiments.

FIG. 10 illustrates a control system for implementing a control strategyin accordance with some embodiments.

FIG. 11 is a flowchart of a control strategy corresponding to thecontrol system illustrated in FIG. 10.

FIG. 12 is schematic diagram of a control loop of the stator voltage inaccordance with some embodiments.

FIG. 13 illustrates another control system for implementing anothercontrol strategy in accordance with some embodiments.

FIGS. 14A and 14B are flowcharts of another control strategycorresponding to the control system in FIG. 13.

FIG. 15 is schematic diagram of a control loop of the stator voltage inaccordance with some embodiments.

FIG. 16 illustrates another control system for implementing anothercontrol strategy in accordance with some embodiments.

FIG. 17 is a flowchart of another control strategy corresponding to thecontrol system in FIG. 16.

FIG. 18 is schematic diagram of a control loop of the stator voltage inaccordance with some embodiments.

FIG. 19 illustrates a waveform of an excitation current of the rotor inaccordance with some embodiments.

FIG. 20 illustrates a waveform of a current and a waveform of a voltageoutput by the apparatus in accordance with some embodiments.

DETAILED DESCRIPTION

References are made to the accompanying drawings that form a parthereof, and in which is shown by way of illustration of the embodimentsin which the embodiments may be practiced. Similar reference numbersindicate similar parts in all diagrams.

FIG. 1 is a schematic diagram of an off-grid power generating apparatusin accordance with some embodiments. An electrical load 140 may be anelectrical appliance, e.g., an oven or an audio player that people usewhile doing camping and electrical appliances used during blackout. Theoff-grid apparatus includes an engine 110, an alternator and excitationcontrol device 170. The engine 110 may receive fuel such as gasoline,diesel, natural gas, or liquid propane vapor through an intake. The fuelprovided to the engine 110 is compressed and ignited to generatereciprocating motion of pistons of the engine 110. The reciprocatingmotion of the piston of the engine 110 is converted to rotary motion bya crankshaft of the engine 110, which is operatively coupled to thealternator. The alternator may be an induction alternator with avariable speed and constant frequency. Specifically, the alternatorincludes a rotor 120 and a stator 130. The rotor 120 may be acylindrical rotor having a plurality of symmetric phase windings. It isreceived within the stator 130 and coaxially coupled with the crankshaftof the engine 110 through a common shaft 121. As the engine 110 rotates,the common shaft 121 drives the rotor 120 to rotate and a rotatingmagnetic field is established in the symmetric phase windings of therotor 120. The excitation control device 170 controls and monitors thealternator and the engine 110, which will be described with more detailsbelow.

In accordance with some embodiments, the stator 130 of the alternatorincludes a single phase winding in which a single phase voltage isinduced responsive to the rotation of the magnetic field established inthe plurality of symmetric phase windings of the rotor 120. The inducedvoltage may be delivered directly as an output voltage to the electricalload 140 via an electrical circuit in accordance with some embodiments.The electrical circuit includes power lines L120, N120, L240 and N240and other electrical parts such as switch 150 illustrated in FIG. 1. Theelectrical circuit may include a wire and a plug for connecting thesingle phase winding and the electrical load. Protection mechanisms suchas a circuit breaker may be provided in the electrical circuit forbreaking the circuit when it is overpowered. The electrical circuit mayalso include an auto transfer system for switching between differentoutput voltages. The electrical circuit herein does not include afrequency conversion device such as a converter and an inverter thatchanges the frequency of the voltage in accordance with someembodiments. In other words, the electrical circuit is configured in away that the induced voltage generated in the single phase winding andthe output voltage applied to the load are at the same frequency. Theexcitation control device 170 modulates the frequency of the inducedvoltage so that the frequency remains at a predetermined range, forexample, 60 hertz (Hz) in North America and 50 Hz in China. Accordingly,the induced voltage is applicable to most electric equipment, which isdesigned with a fixed nominal frequency. In this manner, the frequencyof the output voltage also remains at the predetermined range, since theinduced voltage and the output voltage are at the same frequency.

The single phase winding of the stator 130 may provide not only theoutput voltage, but also an excitation voltage to the phase windings ofthe rotor 120 in accordance with some embodiments. The output voltageprovided to the electric load from the stator side tends to vary whenthe engine speed changes. The output voltage is induced and affected bythe magnetic field induced in the rotor windings. To prevent the outputvoltage from changing, the current in the phase windings of the rotor isregulated to offset the variation tendency of the induced voltage. Inthis manner, the amplitude and the frequency of the output voltageoutput from the stator 130 are kept stable.

The single phase winding of the stator 130 may include an output portion130A and an excitation portion 130B as illustrated in FIG. 1 inaccordance with some embodiments. The output portion 130A and theexcitation portion 130B include at least one coil respectively. Theoutput portion 130A and the excitation portion 130B provide the outputvoltage to the load and the excitation voltage to the rotor windingsrespectively in accordance with some embodiments.

The rotor 120 may include a plurality of symmetric phase windings. FIG.2A illustrates an arrangement of the rotor 220A in accordance with someembodiments. The rotor 220A has three phase windings U, V and W in astar connection. The axes of the three phase windings U, V and W arespatially separated at an angular degree of 120°. Each of the threephase windings U, V and W may include one coil with an equal number ofcoil turns. Terminal of the coils may be connected via binding posts224U, 224V and 224W. Other symmetric configurations of three phasewindings such as a delta connection are also applicable to the powergenerating apparatus. FIG. 2B illustrates another arrangement of therotor in accordance with some embodiments. The rotor 220B includes fivephase windings M, N, O, P and Q arranged in a symmetric manner, i.e.,the axes of the five phase windings M, N, O, P and Q are spatiallyseparated at an angular degree of 72°. Each of the five phase windingsM, N, O, P and Q may include one coil with an equal number of coilturns. Terminal of the coils may be connected via binding posts 220O,220P, 220Q, 220M and 220N. FIG. 2C illustrates another arrangement ofthe rotor 220C in accordance with some embodiments. Two symmetric phasewindings R and S which are separate spatially at an angular degree of90° are provided. Each of the two phase windings R and S may include onecoil with an equal number of coil turns. Terminals of the coils may beconnected via binding posts 220R, 220S and 220J.

In accordance with some embodiments, the alternator includes a voltagesensor for measuring the amplitude of the output voltage U_(LOAD) outputfrom the single phase winding of the stator, a current sensor formeasuring the amplitude of the alternating current I_(LOAD) applied tothe electrical load, and a speed sensor for measuring the rotating speedof the rotor or the engine ω_(r) (not illustrated in FIG. 1 but in FIGS.10, 13 and 16). The amplitudes of the output voltage and the alternatingcurrent and the rotating speed can be real time values measured by thesensors, for example, real time values measured every millisecond orevery second. They can also be the averages or integrals of some realtime values measured by the sensors in a period. For example, thesensors measure real time values every second, and the amplitudes of theoutput voltage and the load current and the rotating speed can be theaverages or integrals of the measured real time values in every 10successive seconds. The measured operation data of the apparatus fromthe current sensor, voltage sensor and speed sensor can be modulated,filtered and then sent to the excitation control device 170. Amicroprocessor-based or otherwise computer-driven system can work as theexcitation control device 170. The excitation control device 170 has aprocessor and a memory for storing program instructions for implementingthe control functions of the excitation device. The processor operatesunder the direction of the stored program instructions. Optionally, theexcitation control device 170 may be provided with a logic circuit suchas CMOS (Complementary Metal Oxide Semiconductor), ASIC (ApplicationSpecific Integrated Circuits), PGA (Programmable Gate Array), FPGA(Field-programmable Gate Array) and so on for implementing its controlfunctions.

As illustrated in FIG. 3, the excitation control device 170 has a firstcalculating element 171, a first modulating element 172, a secondcalculating element 173, a third calculating element 174 and a secondmodulating element 175 in accordance with some embodiments. The firstcalculating element 171 calculates the load power of the electrical loadat least in accordance with the measured amplitudes of the outputvoltage U_(LOAD) and the alternating current I_(LOAD) The firstmodulating element 172 obtains the desired rotation speed of the engineat least in accordance with the calculated load power, and modulates thespeed of the engine at least in accordance with the desired rotationspeed of the engine. The second calculating element 173 determines aslip angle at least in accordance with the rotation speed of the rotoror the engine ω_(r) measured by the speed sensor. The third calculatingelement 174 determines the target voltage of the rotor at least inaccordance with the amplitude of the output voltage measured by thevoltage sensor. The second modulating element 175 generates a modulatingsignal at least in accordance with the target voltage of the rotor andthe slip angle, and modulates the frequency and amplitude of theexcitation current in the phase windings of the rotor with themodulating signal.

Reference is now made back to FIG. 1. The excitation control devices 170is electrically connected with the engine 110. The control device 170may be physically attached to the alternator, and connected with theengine 110 via a wire or a wireless device. The excitation controldevice 170 collects operation data measured by the voltage sensor,current sensor and speed sensor, and modulates the engine speed and thefrequency and amplitude of the excitation current in the phase windingsof the rotor in response to the operation data. While the excitationcontrol device 170 directly controls the engine 110 in the embodimentsdescribed above, the engine 110 may also be directly controlled by anengine control module (ECM) not shown in the figures. The ECM controlsthe engine speed, and thereby controls the output power of thealternator. The excitation control device 170 controls the ECM. The ECMmay be physically attached to the engine 110. A communication bus isprovided between the excitation control device 170 and the ECM fortransmitting communication data between them. In addition to the enginespeed, the ECM may also monitors a variety of engine characteristicssuch as fuel consumption, engine start information and oil pressure.

The output portion has more than one coil to provide an output voltageat dual levels in accordance with some embodiments. The output portion130A includes a first segment 130A1 and a second segment 130A2 inFIG. 1. The first segment 130A1 includes one coil with a first terminalline UAL, which may be a live line, and a second terminal line UAN,which may be a neutral line. The second segment 130A2 may includeanother coil with a third terminal line UBL and a fourth terminal lineUBN, which may be a live line and a neutral line respectively. The firstsegment 130A1 and the second segment 130A2 may separately include aplurality of coils in series in accordance with some embodiments.

The terminal lines UAL, UAN, UBL and UBN of the first segment 130A1 andthe second segment 130A2 are separately connected to the switch 150,which, in turn, is connected with the electrical load 140. A user mayswitch over the switch 150 to selectively connect the first segment130A1 and the second segment 130A2 in parallel or in series. In thismanner, an output voltage at dual levels, for example, 120 volts or 240volts, which are the most commonly used nominal voltages in NorthAmerica, may be generated and delivered from the output portion 130A tothe load via the switch 150.

The power generating apparatus includes a switch 150 in accordance withsome embodiments as shown in FIG. 1. The switch 150 may be a manuallyoperated changeover switch or similar switches. The terminal lines UAL,UAN, UBL and UBN of the first segment 130A1 and the second segment 130A2are separately connected to the switch 150 in accordance with someembodiments. A user may switch over the switch 150 to selectivelyconnect the first segment 130A1 and the second segment 130A2 in parallelor in series. In this manner, an output voltage at dual levels, a lowvoltage (for example, 120 volts) and a high voltage (for example, 240volts), may be generated and delivered from the output portion 130A tothe load through the switch 150. Optionally, a plug connected with theelectric load and two sockets connected with the first segment 130A1 andthe second segment 130A2 may be used by a user to switch between the lowvoltage and the high voltage. For example, the user may put the plug inone socket through which the first segment 130A1 and the second segment130A2 are in series connection to provide the electric load with thehigh voltage. The user may put the plug in the other socket throughwhich the first segment 130A1 and the second segment 130A2 are inparallel connection (or either of the two segments connected) to providethe electric load with the low voltage. In some embodiments, the switch150 (including any sensor associated with the switch 150) may send asignal to the excitation control device, indicating whether the one orboth of the segments are connected or whether the connection is inparallel or in series.

The excitation portion 130B may include one coil that has terminal linesconnected to the phase windings of the rotor U, V and W to apply theinduced voltage generated in the excitation portion 130B to the phasewindings for energizing the rotating magnetic field. The excitationportion 130B, independent of the output portion 130A, can provide anexcitation voltage greater than the output voltage in a simple manner.For example, an excitation voltage of 320 volts (higher than a normal220 volts) can be provided when the excitation portion 130B has asufficient number of winding turns. The stator of the apparatus may haveonly one coil in total in accordance with some embodiments. An outputvoltage at one single level is generated and delivered to the electricalload from the stator. This stator operates in a way similar to the waywhen the output portion 130A and the excitation portion 130B illustratedin FIG. 1 are permanently connected together. Those skilled in the artwould understand how the stator with one single coil operates withreference to the description above.

FIG. 4 is a schematic diagram of another off-grid power generatingapparatus in accordance with some embodiments. The output portion 430Aof the stator winding has only one coil in which an output voltage atone single level, e.g., 120 volts or 240 volts, is generated. The outputvoltage is provided to the electrical load 440 via a circuit, whichincludes the live line and the neutral line in FIG. 4.

FIG. 5 is a schematic diagram of yet another off-grid power generatingapparatus in accordance with some embodiments. Similar reference numbersare used in FIGS. 1, 4 and 5 to indicate similar structural parts. Forexample, the reference numbers of 170, 470 and 570 in FIGS. 1, 4 and 5all refer to the excitation control device for controlling andmonitoring the alternator and the engine. The single phase winding ofthe stator 530 may include a first portion 530A1 and a second portion530A2 in accordance with some embodiments. The first portion 530A1 mayinclude one coil with terminal lines UAL and UAN, and the second portion530A2 may include another coil with terminal lines UBL and UBN. Theterminal lines UAL, UAN, UBL and UBN are separately connected to theswitch 550. A user may switch over the switch 550 to selectively connectthe first portion 530A1 and the second portion 530A2 in parallel or inseries to obtain an output voltage of 120 volts or 240 volts. Unlike theapparatus illustrated in FIG. 1 that has a separate excitation portion,the first portion 530A1 or the second portion 530A2 of this apparatus isalso used to provide the excitation voltage. For example, as illustratedin FIG. 5, the second portion 530A2 has an extra pair of terminal linesEXN and EXL that are connected to the phase windings of the rotor (notillustrated in FIG. 5).

The second portion 530A2 may include more than one coil in seriesconnection in accordance with some embodiments (not illustrated in FIG.5), for example, a first coil and a second coil connected in series. Thelive line and neutral line of the first coil are connected to the switchto provide the output voltage, and the live lines of the first coil andthe second coil are connected to the rotor circuit, i.e., the seriesvoltage generated by the first coil and the second coil in seriesconnection works as the excitation voltage. Compared with a stator witha separate excitation coil, the coil number of the excitation coil inthis stator is reduced.

A frequency conversion device 560 may be provided between the secondportion 530A2 and the phase windings of the rotor in accordance withsome embodiments. The frequency conversion device 560, which is theso-called converter, regulates the induced voltage generated in theexcitation portion 530A2 to generate a voltage with a desired frequencyand amplitude to provide to the phase windings of the rotor forenergizing the rotating magnetic field. The frequency conversion device560 may include a DC bus voltage regulator, which receives theexcitation voltage from the single phase winding of the stator andoutputs a DC voltage to buses BUS+, BUS−. FIG. 6A illustrates a DC busvoltage regulator 661A in accordance with some embodiments. The DC busvoltage regulator 661A includes an uncontrolled Bridge Rectifier 661Athat may have four individual rectifying diodes 661A1 connected in“bridge” configuration to receive the excitation voltage from theexcitation portion 630B or either of the two portions of the singlephase winding and generate the desired DC voltage. The main advantage ofthis bridge rectifier is that it does not require a special centertapped transformer, thereby reducing its size and cost. A bus capacitor661A2 may be provided for smoothing the output of the uncontrolledBridge Rectifier 661A to produce a DC voltage. FIG. 6B illustratesanother DC bus voltage regulator 661B in accordance with otherembodiments. A Power factor correction device 661B1 is used to rectifyand boost the excitation voltage received from the single phase windingof the stator. A bus capacitor 661B2 may be provided to store energy andfilter out high frequency voltage components.

FIG. 7 illustrates another embodiment of providing an excitation voltageto the phase windings of the rotor in accordance with some embodiments.A DC power source, which may be a battery 710, is used for supplying aDC voltage. A DC-DC converter 720 is connected with the battery 710 toincrease the amplitude of the DC voltage before the DC voltage isapplied to the buses BUS+ and BUS−, which is connected, to the phasewindings of the rotor for energizing the rotating magnetic field. A buscapacitor 730 may be provided to store energy and filter out highfrequency voltage components.

FIG. 8 is a schematic diagram of another off-grid power generatingapparatus in accordance with some embodiments. This exemplary apparatusis similar to those described previously with reference to FIGS. 1 and5, and therefore identical and similar parts will not be discussed againherein. The apparatus includes a frequency conversion device 860. Thefrequency conversion device 860 mainly includes a DC bus voltageregulator 861 and an inverter 862. The DC bus voltage regulator 861 issimilar to those described above with reference to FIGS. 6A and 6B, andtherefore the frequency conversion device will not be described againherein.

The inverter 862 can be a two-phase, three-phase, four-phase orfive-phase DC/AC inverter corresponding to the number of the symmetricphase windings of the rotor. In FIG. 8, the inverter 862 is illustratedas a three-phase six-switch DC/AC inverter, which receives control pulsesignals, for example, in the form of a PWM (pulse-width modulation) orSVPWM (space vector pulse width modulation) waveform from the excitationcontrol device 870. The control pulse signals are duty ratios ofswitching-on time in substance. The excitation control device 870 isprogrammed to provide the pulse signals, which are PWM or SVPWMexcitation signals with a desired amplitude and frequency. To keep theinduced voltage constant, the pulse signals are employed to modulate theamplitude and frequency of the current in the phase windings of therotor, in order to change the magnetic field in a way that offsets thevariation tendency of the induced voltage. The excitation control device870 applies the excitation signals to the rotor windings to regulate theintensity of the rotating magnetic field generated in the rotor windingsand the rotating speed of the rotating magnetic field relative to therotor. As a result, the amplitude and frequency of the output voltageoutput from the stator, i.e., the output voltage of the apparatus, arekept stable. The PWM or SVPWM waveform can be a square wave, modifiedsine wave and sine wave mainly depending on the circuit design of theinverter 862. Each leg of the inverter 862 may be connected with onephase winding of the rotor through wires. The inverter 862 has one ormore switching element on each leg. The switching element can be asemiconductor switching element such as IGBT, BJT, MOSFET, GTO, SCR andIGCT. A pair of IGBTs is provided on each leg of the inverter 862 asillustrated in FIG. 8. The pulse signals from the excitation controldevice 870 successively control the switch ON and OFF time of theswitching elements with duty ratios. A relatively stable DC voltage fromthe DC bus voltage regulator 861 is applied to the inverter 862 asillustrated in FIG. 8. The required input DC voltage of the inverter 862mainly depends on the design and function of the inverter 862. Factorsthat can be considered include the amplitude of the induced voltage, therotating speed range of the engine, the structure of the rotor windings,the current and voltage parameters of IGBT, etc. The inverter 862generates PWM or SVPWM excitation signals with a desired frequency andamplitude, and the excitation signals are employed to modulate theamplitude and frequency of the current in the phase windings of therotor so that a rotating magnetic field with a desired intensity androtating speed relative to the rotor is established.

The frequency conversion device 860, particularly the inverter 862, isdisposed on the rotor side in the embodiments. In other words, theinduced voltage generated in the single phase winding of the stator isdelivered to the electrical load without going through any frequencyconversion device. The induced voltage and the output voltage applied tothe load are at the same frequency. As a result, the rated power of theinverter 862 used in the apparatus with a rated power of 7,000 watts isless than 1,500 watts, usually the rated power of the inverter 862 is700-800 watts. The rated power of an inverter in a power generator witha rated power of 7,000 watts in which an inverter is used to modulatethe overall power generated by the generator is usually 7000 watts.Thus, inverters with a much smaller capacity can be used in theapparatus in the embodiments, since the inverter 862 only modulates afraction of the overall power that is supplied to the rotor formodulating the amplitude and frequency of the output voltage of theapparatus output from the stator side. Accordingly, the inverter 862 islighter and more cost-efficient. It is estimated that the cost of theinverter accounts for 20% to 60% of the cost of a traditional powergenerator. Therefore, the power generating apparatus in the embodimentsdescribed above enjoys a superb advantage in terms of costs.

The excitation control device 870 is programmed to calculate the loadpower of the engine by using the operation data measured by the currentsensor and the voltage sensor (not shown in FIG. 8) and modulate thepower of the engine, i.e., the speed of the engine in response to thecalculated load power. The power of the engine is modulated to follow apre-defined engine power-speed characteristic cure of the engine totrack the maximum power point. The characteristic curve of the engine isa curve indicating a relationship of operation parameters of the engine,for example, the power, rotation torque and rotation speed of theengine. The characteristic curve can be pre-stored in the excitationcontrol device 870.

The alternator may also include a battery 880 in accordance with someembodiments. The battery 880 may be electrically separated from thebuses BUS+, BUS− for the sake of safety. The DC voltage of the battery880 may be applied to the buses BUS+, BUS− through an excitation voltageprovider 890 and provides an excitation voltage for establishing arotating magnetic field in the rotor windings when the power generatingapparatus starts. The excitation voltage provider 890 may be structuredin the form of a transformer. The amplitude of the excitation voltagemay be quite small, for example from 1 volts to 20 volts.

The apparatus is set to operate at a rotation speed equal to or lessthan the synchronous speed of the alternator in accordance with someembodiments. For example, when the synchronous speed of the alternatoris 3600 rpm, the engine (e.g., a gasoline engine) is set to operate at aspeed between 3000-3600 rpm in a stable working state. The faster theengine runs, the greater the output power of the engine is. The enginespeed of a power generating apparatus will increase up to 3600 rpm asthe apparatus picks up its power from an idling state when the apparatusstarts to its rated power. When the rotating speed of the engine reaches3600 rpm, the excitation voltage becomes a DC voltage. Thus, anapparatus will always operate in a sub-synchronous or synchronous statewhen the apparatus is set to operate at a rotation speed equal to orless than the synchronous speed of the alternator. The synchronous speedof the alternator refers to the rotation rate of the stator's magneticfield. This means that electrical energy in the apparatus always flowsfrom the stator side to the rotor side. No energy flows in the oppositedirection. This feature makes it possible to use low cost devices orparts with a uni-directional characteristic such as the uncontrolledbridge rectifier illustrated in FIG. 6A in the apparatus. It should beappreciated that the apparatus may operate at a speed more than thereference speed in undesirable operation conditions. Protectionmechanisms such as a circuit breaker may be provided in the apparatus tostop it from operation when the speed is excessive.

FIG. 9 is a schematic diagram of yet another off-grid power generatingapparatus in accordance with some embodiments. The inverter 962 can betwo-phase, three-phase, four-phase or five-phase DC/AC inverterscorresponding to the number of the symmetric phase windings of therotor. In FIG. 9, the inverter 962 is illustrated as a three-phasesix-switch DC/AC inverter that receives control pulse signals, forexample, in the form of a PWM (pulse-width modulation) or SVPWM (SpaceVector Pulse Width Modulation) waveform from the excitation controldevice 970. The control pulse signals are duty ratios of switching-ontime in substance. The excitation control device 970 is programmed toprovide the pulse signals that are PWM or SVPWM excitation signals witha desired amplitude and frequency. The pulse signals are employed tomodulate the amplitude and frequency of the current in the phasewindings of the rotor to offset the variation tendency of the amplitudeand frequency of the induced voltage generated in the single phasewinding of the stator, which is caused by the speed variations of theengine corresponding to load changes. The excitation control device 970applies the excitation signals to the rotor windings to regulate theintensity of the rotating magnetic field generated in the rotor windingsand the rotating speed of the rotating magnetic field relative to therotor. As a result, the amplitude of the output voltage of the stator,i.e., the output voltage of the apparatus, is kept stable and thefrequency of the output voltage is maintained constant. The PWM/SVPWMwaveform can be a square wave, modified sine wave and sine wavedepending on the circuit design of the inverter 962. Each leg of theinverter 962 may be connected with one phase winding of the rotorthrough wires. The inverter 962 has one or more switching elements oneach leg. The switching elements can be semiconductor switching elementssuch as IGBT, BJT, MOSFET, GTO, SCR and IGCT. A pair of IGBTs isprovided on each leg of the inverter 962 in accordance with someembodiments as illustrated in FIG. 9. The pulse signals from theexcitation control device 970 successively control the switch ON and OFFtime of the switching elements of the inverter 962 with duty ratios. Arelatively stable DC voltage from the DC bus voltage regulator 961 isapplied to the inverter 962 as illustrated in FIG. 9. The required inputDC voltage of the inverter 962 depends on the design and function of theinverter 962. Factors that can be considered include the amplitude ofthe induced voltage, the rotating speed range of the engine, thestructure of the rotor windings, the current and voltage parameters ofIGBT used, etc. The inverter 962 generates PWM or SVPWM excitationsignals with desired frequency and amplitude, and the excitation signalsare employed to modulate the amplitude and frequency of the current inthe phase windings of the rotor so that a rotating magnetic field withthe desired intensity and rotating speed relative to the rotor isestablished. The rated power of the inverter 962 used in the apparatuswith a rated power of 7,000 watts is less than 1,500 watts, usually is700-800 watts. The rated power of an inverter in a power generator witha rated power of 7,000 watts in which an inverter is used to regulatethe overall power generated by the generator is usually 7000 watts.Thus, inverters with a much smaller capacity can be used in theapparatus in the embodiments, since the inverter 962 only regulates afraction of the overall power that is supplied to the rotor forregulating the amplitude and frequency of the output voltage of theapparatus output from the stator side. Accordingly, the inverter 962 iscomparatively light and cost-efficient. It is estimated that the cost ofthe inverter accounts for 20% to 60% of the cost of a traditional powergenerator. Therefore, the power generating apparatus in the embodimentsdescribed above enjoys a superb advantage in terms of costs.

The excitation control device 970 is programmed to regulate the power ofthe engine, i.e., the speed of the engine. The excitation control device970 calculates real time load power with the measured operation datafrom the sensors that are not illustrated in FIG. 9, and regulates speedof the engine in response to the calculated real time load power. Thepower of the engine is regulated to follow a pre-defined enginepower-speed characteristic of the engine to track the maximum powerpoint. The pre-defined engine power-speed characteristic of the enginecan be stored in the excitation control device 970.

The alternator may also include a battery 980 in accordance with someembodiments. The battery 980 may be electrically separate from the busesBUS+, BUS− for the sake of safety. The DC voltage of the battery 980 maybe applied to the buses BUS+, BUS− through an excitation voltageprovider 990 for providing an excitation voltage for establishing arotating magnetic field in the rotor windings when the power generatingapparatus starts. The excitation voltage provider 990 may be structuredin the form of a transformer. The amplitude of the excitation voltagemay be quite small, for example from 1 volts to 20 volts.

The difference between FIG. 8 and FIG. 9 is that the apparatus in FIG. 9does not have a separate excitation portion as the apparatus illustratedin FIG. 5. It is easy for those skilled in the art to understand theprinciple of the apparatus in FIG. 9 thus no more description will berepeated herein.

The power generating apparatus for powering an electrical load isdescribed with reference to FIGS. 1-9 above. The apparatus, aspreviously described, having a stator with a single phase winding and arotor with a plurality of symmetric phase windings poses a challenge toconventional control strategies for generators. It is difficult tocontrol an apparatus having a stator with a single phase winding inwhich an output voltage is generated and output without frequencyconversion. A simple and cost-effective control system for the apparatusis provided in this disclosure, which will be described with referenceto FIGS. 10-18.

In the control system, an excitation signal is employed to modulate thefrequency and amplitude of the current in the rotor windings. Therebythe intensity of the rotating magnetic field and its rotating speedrelative to the rotor are modulated. In this manner, the amplitude andfrequency of the output voltage are kept stable. Meanwhile, the rotatingspeed of the engine is variable in response to load changes so that thefuel efficiency of the engine is optimized. This control system enablesthe power generating apparatus to change its engine speed in a widerange as the load it drives changes.

FIG. 10 illustrates a control system for implementing a control strategyin accordance with some embodiments. FIG. 11 is a flowchart of a controlstrategy corresponding to the control system illustrated in FIG. 10. Thesingle phase winding of the stator in these embodiments generates anoutput voltage of one level. The one leveled output voltage may be liveto live high voltage of 240 volt U_(SAB) which is provided to theelectrical load through live terminal lines A and B to the load (asillustrated in FIG. 10). Another example of the one leveled outputvoltage is live to neutral 120 volt low voltage which is provided to theelectrical load through live terminal line A and neutral terminal line Nto the load (not illustrated in FIG. 10). In accordance with someembodiments, the level of the output voltage can be changed byregulating the intensity of the rotating magnetic field. For instance, alow output voltage of 120 volts can be boosted to a high output voltageof 240 volts by increasing the intensity of the rotating magnetic field.It should be understood that the control system illustrated in FIG. 10and the control strategy illustrated in FIG. 11 are applicable to anapparatus that generates an output voltage of 120 volts from the statorside.

The rotor has three windings U, V, W in a symmetric configuration thatare connected with three legs of an inverter 1062 separately. A DCvoltage U_(dc) from the DC bus voltage regulator (which is notillustrated FIG. 10) is applied to the inverter 1062 as previouslydescribed. A voltage sensor 10V and a current sensor 10A areelectrically connected with the terminal lines A and B for measuring theoutput voltage U_(SAB) and the load current I_(SAB) output by the singlephase winding of the stator. Specifically, the amplitude of the outputvoltage U_(SAB) and the load current I_(SAB) are measured. A speedsensor 10S is connected with the rotor to measure the rotating speed ofthe rotor. The speed sensor 10S may measure the rotating speed of theengine, since the rotor and the engine are coaxially connected. Theamplitudes of the output voltage and the load current and the rotatingspeed can be real time values measured by the voltage sensor 10V, thecurrent sensor 10A and the speed sensor 10S, for example, real timevalues measured every millisecond or every second. They can also be theaverages or integrals of some real time values measured by the sensorsin a period. For example, the sensors measure real time values everysecond, and the amplitudes of the output voltage and the load currentand the rotating speed can be the averages or integrals of the measuredreal time values in every 10 successive seconds. The measured operationdata of the apparatus from the current sensor 10A, voltage sensor 10Vand speed sensor 10S may be modulated and filtered and then sent to theexcitation control device 1070.

When the excitation control device 1070 collects the measured operationdata, load power P_(load) is calculated using equation 1 below:P _(load) =U _(SAB) *I _(SAB)  Equation 1

Where U_(SAB) is the amplitude of the measured output voltage andI_(SAB) is the amplitude of the measured load current. In accordancewith some embodiments, a desired rotation speed of the engine can bedetermined using the pre-defined characteristic curve of the engine. Thecharacteristic curve of the engine is a curve indicating a relationshipof operation parameters of the engine, for example, the power, rotationtorque and rotation speed of the engine. The characteristic curve can beobtained with experiments and pre-stored in the excitation controldevice. For a certain load, the excitation control device identifies thecorresponding desired rotation speed on the characteristic curve of theengine. In some embodiments, the characteristic curve can also be atable or a formula describing the corresponding relationships betweenthe load and the optimal rotation speed (sometimes with other parameterssuch as desired voltage as well). When the desired rotation of theengine is available, the excitation control device can correct therotation speed of the engine by using the desired rotation speed with aclosed loop to optimize fuel consumption in response to load changes.

A slip angle θ_(slip) can be determined using equations 2 and 3 below:ω_(slip)=ω₁−ω_(r)  Equation 2θ_(slip)=∫ω_(slip)  Equation 3Where ω_(r) is the rotation speed of the rotor, and ω₁ is thesynchronous speed of the alternator.

In the control strategy illustrated in FIG. 11, a voltage sensor and acurrent sensor respectively measure the output voltage U_(SAB) and theload current I_(SAB) output by the single phase winding of the stator. Aspeed sensor measures the rotation speed of the rotor. The excitationcontrol device calculates the load power with the equation ofP_(load)=U_(SAB)*I_(SAB), and determines the desired engine speedaccording to the load power P_(load). The excitation control device (orECM) modulates the engine speed according to the desired rotation speedof the engine with a close control loop. The excitation control devicedetermines the slip angle θ_(slip) at least according to the rotationspeed of the rotor, and the target voltage of the rotor Urq* at leastaccording to the measured amplitude of the output voltage. Then theexcitation control device generates a modulating signal according to thetarget voltage of the rotor Urq* and the slip angle and modulates thefrequency and the amplitude of the current in the phase windings of therotor with the modulating signal. It should be noted that the controlstrategy illustrated in FIG. 11 is also applicable to an apparatus thatoutputs a live to neutral voltage from the stator side.

FIG. 12 is schematic diagram of a control loop of the stator voltage inaccordance with some embodiments. A closed control loop is used toautomatically correct the amplitude of the target voltage of the rotorso that the amplitude of the output voltage is kept constant. AProportional-Integral (PI) regulator is provided in the closed controlloop to reduce errors of the target voltage of the rotor. The outputvoltage U_(SAB), which is a negative feedback, and the target voltage ofthe stator U_(s)* work as inputs of the PI regulator. The output of thePI regulator is the target voltage of the rotor U_(rq)*. The obtainedslip angle θ_(slip) and target voltage of the rotor U_(rq)*are used bythe excitation control device to generate a pulse signal with a certainduty ratio, which is input into the inverter as illustrated in FIG. 10to regulate the switch ON and OFF time of the switching elements of theinverter. The inverter regulates the amplitude and frequency of thecurrent in the rotor windings. Thereby the intensity of the rotatingmagnetic field established in the rotor windings and the rotating speedof the rotating magnetic field relative to the rotor are regulated sothat the amplitude and frequency of the induced voltage generated in thestator winding are kept constant.

FIG. 13 illustrates another control system for implementing anothercontrol strategy in accordance with some embodiments. FIG. 14 is aflowchart of another control strategy corresponding to the controlsystem in FIG. 13. FIG. 15 is schematic diagram of a control loop of thestator voltage in accordance with some embodiments. The differencesbetween this control system and the previous one illustrated in FIGS.10-12 are as follows.

As illustrated in FIG. 13, the single phase winding of the statoroutputs to the electrical load a dual output voltage, i.e., a highvoltage and a low voltage, through live terminal lines A, B and neutralterminal line N. The high voltage is live to live voltage Us_(AB), andthe low voltage is live to neutral voltage U_(AN) and U_(BN). A user canswitch over the switch as described previously to select the highvoltage or the low voltage. The switching over signal can be deliveredto the excitation device 1370 via a signal line or wirelesscommunications devices such as WIFI devices. The excitation device 1370selects the high voltage or the low voltage responsive to the switchingover signal it receives. A first current sensor 13A1 and a secondcurrent sensor 13A2 are provided to measure the first live to neutralcurrent I_(BN) and the second live to neutral current I_(AN). Likewise,a first voltage sensor 13V1 and a second voltage sensor 13V2 areprovided to measure the first live to neutral voltage U_(BN) and thesecond live to neutral voltage U_(AN). Additionally, a third voltagesensor 13V3 is provided to measure the high voltage U_(SAB).

When the apparatus operates in the low voltage mode, the first and thesecond live to neutral load powers P_(load AN) P_(load BN) arecalculated using equations 4 and 5 below:P _(load AN) =U _(AN) *I _(AN)  Equation 4P _(load BN) =U _(BN) *I _(BN)  Equation 5

Where U_(AN) and U_(BN) are the amplitudes of the measured live toneutral output voltages, and I_(AN) and I_(BN) are the amplitude of themeasured load current. Then the total power P_(load total) is calculatedwith equation 6 below:P _(load total) =P _(load AN) +P _(load BN)  Equation 6

Where P_(load AN) and P_(load BN) are the first and second live toneutral load powers. The excitation device then determines the desiredengine speed in accordance with the total power P_(load total).

When the apparatus operates in the high voltage mode, a load powerP_(load) is calculated using equation 7 below:P _(load) =U _(SAB) *I _(AN)  Equation 7

Where U_(SAB) is the amplitude of the measured output voltage and I_(AN)is the amplitude of the measured load current. I_(BN) can be used toreplace I_(AN) in equation 7 since I_(AN) and I_(BN) are equal in thehigh voltage mode. The excitation device then determines the desiredengine speed in accordance with the load power P_(load).

In accordance with some embodiments, the control strategy distinguishesthe working mode of the system. The system may operate in a high voltagemode and a low voltage mode. FIG. 14A and FIG. 14B respectivelyillustrate the flowcharts of the low voltage mode and the high voltagemode. The excitation control device receives a working mode signal fromthe switch. The working mode signal indicates whether the apparatus isworking on the high voltage mode or low voltage mode. One or morevoltage sensors measure the amplitudes of the low voltage U_(AN) andU_(BN), the high voltage U_(SAB), One or more current sensors measurethe live to neutral currents I_(AN) and I_(BN). A speed sensor measuresthe rotation speed of the rotor. In the low voltage mode as illustratedin FIG. 14A, the excitation control device calculates the total loadpower with the equation of P_(load total)=U_(AN)*I_(AN)+U_(BN)*I_(BN),and determines the desired engine speed according to the total loadpower P_(load total). The excitation control device (or ECM) modulatesthe engine speed according to the desired rotation speed of the enginewith a close control loop. The excitation control device determines theslip angle θ_(slip) at least according to the rotation speed of therotor, and the target voltage of the rotor U_(rq)* according to theamplitude of the measured low voltage U_(AN) or U_(BN). Then theexcitation control device generates a modulating signal according to thetarget voltage of the rotor U_(rq)* and the slip angle θ_(slip), anduses the modulating signal to modulate the frequency and the amplitudeof the current in the phase windings of the rotor. FIG. 14B illustratesthe flowchart of the high voltage mode. The excitation control devicecalculates the load power with the equation of P_(load)=U_(SAB)*I_(AN).In some embodiments, I_(BN) can be used to replace I_(AN) since I_(AN)and I_(BN) are equal in the high voltage mode. Then the excitationcontrol device obtains the desired engine speed according to the loadpower P_(load). The excitation control device (or ECM) modulates theengine speed according to the desired rotation speed of the engine witha close control loop. The excitation control device determines the slipangle θ_(slip) at least according to the rotation speed of the rotor,and the target voltage of the rotor Urq* according to the amplitude ofthe measured high voltage U_(SAB). Then the excitation control devicegenerates a modulating signal according to the target voltage of therotor Urq* and the slip angle θ_(slip). The excitation control deviceuses the modulating signal to modulate the frequency and the amplitudeof the current in the phase windings of the rotor.

FIG. 15 is schematic diagram of a control loop of the stator voltage inaccordance with some embodiments. This control loop switches between twomodes, a high voltage mode and a low voltage mode. In the high voltagemode, the high output voltage U_(SAB) is set as a negative feedbackinput to the PI regulator. The target voltage of the stator U_(s)*serves to correct the high voltage U_(SAB) in the PI regulator. Theoutput of the PI regulator is the target voltage of the rotor U_(rq)*.In the low voltage mode, the low output voltage U_(AN) or U_(BN) is setas a negative feedback input to the PI regulator. The target voltage ofthe stator U_(s)* serves to correct the low output voltage U_(SAB) inthe PI regulator. The output of the PI regulator is also the targetvoltage of the rotor U_(rq)* in the low voltage mode. The obtained slipangle and target voltage of the rotor are then used by the excitationcontrol device to generate a pulse signal. Other features in thiscontrol system are similar to those described previously with referenceto FIGS. 10-12, and therefore will not be discussed again herein.

FIG. 16 illustrates another control system for an off-grid powergenerating apparatus in accordance with some embodiments. FIG. 17 is aflowchart of a control strategy corresponding to the control systemillustrated in FIG. 16 in accordance with some embodiments. FIG. 18 isschematic diagram of a control loop of the stator voltage in accordancewith some embodiments. As illustrated in FIG. 16, the stator windingdelivers to the load a live to live voltage U_(SAB) (i.e., the highvoltage) via live line A and B, and a live to neutral voltage U_(AN) orU_(BN) (i.e., low voltage) via live line A or B and neutral line N. Auser can switch over the switch to select the high voltage or the lowvoltage. Unlike the embodiments illustrated in FIGS. 13-15, theexcitation control device 1670 employs the same control strategy inthese embodiments, no matter whether the apparatus operates in the highvoltage mode or the low voltage mode. Thus, switching signals from theswitch are not needed for monitoring the apparatus in these embodiments.

A first current sensor 16A1 and a second current sensor 16A2 areprovided to measure the first live to neutral current I_(BN) and thesecond live to neutral current I_(AN). Likewise, a first voltage sensor16V1 and a second voltage sensor 16V2 are provided to measure the firstlive to neutral voltage U_(BN) and the second live to neutral voltageU_(AN). The live to live voltage (i.e., the high voltage) is not neededfor implementing the control strategy in these embodiments.

In accordance with some embodiments, no matter whether the apparatusoperates in the high voltage mode or the low voltage mode, the loadpower of the apparatus is calculated with equation 8 below:P _(load total) =U _(AN) *I _(AN) +U _(BN) *I _(BN)  Equation 8

Where U_(AN) and U_(BN) are the first live to neutral voltage and thesecond live to neutral voltage, and I_(AN) and I_(BN) are the first liveto neutral current and the second live to neutral current. Theexcitation device then determines the desired engine speed in accordancewith the load power P_(load total).

In accordance with some embodiments, the voltage sensor and the currentsensor measure the amplitudes of the live to neutral output voltageU_(AN) and U_(BN) and the load currents I_(AN) and I_(BN) in the controlstrategy illustrated in FIG. 17. The speed sensor measures the rotationspeed of the rotor. Then the excitation control device calculates thetotal load power P_(load total)=U_(AN)*I_(AN)+U_(BN)*I_(BN), and obtainsthe desired speed of the engine according to the total load power. Theexcitation control device (or ECM) modulates the engine speed accordingto the desired rotation speed of the engine with a close control loop.The excitation control device determines the slip angle θ_(slip) atleast according to the rotation speed of the rotor, and the targetvoltage of the rotor Urq* at least according to the measured amplitudeof the Live to neutral output voltage U_(AN) or U_(BN). Then theexcitation control device generates a modulating signal according to thetarget voltage of the rotor Urq* and the slip angle θ_(slip), andmodulates the frequency and the amplitude of the current in the phasewindings of the rotor with the modulating signal.

FIG. 18 is schematic diagram of a control loop of the stator voltage inaccordance with some embodiments. Unlike the embodiments illustrated inFIGS. 13-15, the control loop of the stator voltage in the embodimentsdoes not switch over between the high voltage mode and the low voltagemode. Rather, the control loop of the stator voltage just employs thelive to neutral voltage U_(AN) or U_(BN) as a negative feedback input tothe PI regulator. The target voltage of the stator U_(s)* serves asanother input of the PI regulator to correct the live to neutral voltageU_(AN) or U_(BN). The output of the PI regulator is the target voltageof the rotor U_(rq)*. The obtained slip angle and target voltage of therotor are used by the excitation control device to generate a pulsesignal with a certain duty ratio, which is input into the inverter 1662illustrated in FIG. 16 to regulate the switch ON and OFF time of theswitching elements of the inverter. The inverter 1662 regulates theamplitude and frequency of the current in the rotor windings. Therebythe intensity of the rotating magnetic field established in the rotorwindings and the rotating speed of the rotating magnetic field relativeto the rotor are regulated so that the amplitude and frequency of theinduced voltage generated in the stator winding are regulatedaccordingly. Only the live to neutral voltage of the stator is monitoredin the embodiments illustrated in FIGS. 16-18 and the excitation controldevice does not need to communicate with the switch to obtain itsoperating mode.

It should be noted that the operations illustrated in FIGS. 11, 14 and17 can be implemented in an order different from the order illustratedin these figures. Some operations can be conducted substantiallysimultaneously or in a reverse order, depending on the functionsachieved by the operations. For example, determining the target voltageof the rotor can be conducted after or at the same time as the step ofmeasuring the rotating speed of the rotor is implemented.

A meter for measuring the angular position of the rotor such as anencoder, which is typically expensive, is not used in the control systemin accordance with some embodiments. Further, a current control loop,which is usually involved in vector control methods, is not used in thesystem. The control system is therefore simple and easy to implement. Asa result, an excitation control device with a low capability can be usedin the apparatus.

FIG. 19 illustrates a waveform of an excitation current for establishinga magnetic field in the rotor windings in accordance with someembodiments. This waveform is achieved by an apparatus with a singlephase stator winding and three phase rotor windings which operates at3000 rpm and outputs 240 volt voltage from the stator side and isapplied with a resistive load of 5 kW (kilowatt). FIG. 20 illustrates awaveform of a current and a waveform of a voltage output from the statorside of the apparatus in accordance with some embodiments. The waveformsare achieved by an apparatus having a stator with a single phasewinding. The apparatus that is applied with a resistive load of 5 kWoperates at a speed of 3000 rpm, and outputs an output voltage of 240volt from the stator side. FIGS. 19 and 20 clearly indicate that thecontrol system yields satisfactory waveforms.

The various embodiments disclosed above have many advantages. Theapparatus has a stator with a single winding and a rotor with aplurality of symmetric windings. The combination of a stator with asingle winding and a rotor with a plurality of symmetric phase windingsenables the apparatus to power single phase electrical devices withsmall rated powers such as household appliances while keeping thecontrol of the apparatus simple and easy.

A meter for measuring the angular position of the rotor such as anencoder, which is typically expensive, is not used in the control systemin accordance with some embodiments. Further, a current control loop,which is usually involved in vector control methods, is not used in thesystem. The control system is therefore simple and easy to implement. Asa result, an excitation control device with a low capability can be usedin the apparatus.

The power generating apparatus outputs electrical power directly fromthe stator without any frequency conversion in accordance with someembodiments. Unlike a generator with an AC-DC-AC converter regulatingthe full power (overall power) of the generator, the apparatus merelymodulates a fraction of its full power with an inverter. The invertermodulates the amplitude and frequency of the current in the rotorwindings to compensate for variations of the induced voltage that isgenerated in the stator winding. In this manner, the amplitude andfrequency of the output voltage from the stator, i.e., the outputvoltage of the apparatus, are kept stable. It is estimated that therated power of a power converter disposed on the rotor side of a powergenerating apparatus accounts for merely around 10% of the rated outputpower of the power generating apparatus. Thus, an inverter with a lowerrated capacity can be used in the apparatus.

The inverter applies an AC voltage to the phase windings of the rotor asan excitation voltage for establishing the rotating magnetic field inthe rotor. Both the amplitude of the excitation voltage and itsfrequency are controllable. This is advantageous given that only theamplitude of the excitation voltage is controllable when a DC voltage isused as the excitation voltage.

The operation speed of the engine in the apparatus is desirablyadjustable to maximize fuel efficiency, and thus reduces CO₂ emissionsof the engine for a given load. Optimizing the operating speed of theengine corresponding to a given load also reduces the noise associatedwith operation of the engine-driven apparatus and extends the life ofthe engine. The output voltage from the stator is substantially kept ata constant frequency.

The amplitude of the output voltage provided by the apparatus is stablegiven that a closed voltage loop is employed to determine the targetrotor voltage for regulating the intensity of the rotating magneticfield. With this feature, the apparatus can be utilized to powerelectrical devices such as audio and video players and some scientificinstruments that are sensitive to voltage and frequency instability.Furthermore, the apparatus can provide an output voltage at dual levelswith a single phase winding so that users can use the apparatus to powerelectrical devices with different nominal voltages.

The apparatus is set to operate at a speed equal to or less than thesynchronous speed of the alternator, which means that electrical energyflows uni-directionally, i.e., from the stator to the rotor, not vicevisa. This feature renders cheap devices such as uncontrolled bridgerectifier applicable to the apparatus. The feature also makes itpossible to control the apparatus with a simple and easy controlstrategy. Devices with a comparatively low capability can also be usedin the apparatus.

The above is only the preferred embodiments of the application and notintended to limit the application, and any modifications, equivalentreplacements, improvements and the like within the spirit and principleof the application shall fall within the scope of protection of theapplication.

While particular embodiments are described above, it will be understoodit is not intended to limit the application to these particularembodiments. On the contrary, the application includes alternatives,modifications and equivalents that are within the spirit and scope ofthe appended claims. Numerous specific details are set forth in order toprovide a thorough understanding of the subject matter presented herein.But it will be apparent to one of ordinary skill in the art that thesubject matter may be practiced without these specific details. In otherinstances, well-known methods, procedures, components, and circuits havenot been described in detail so as not to unnecessarily obscure aspectsof the embodiments.

Although the terms first, second, etc. may be used herein to describevarious elements, these elements should not be limited by these terms.These terms are only used to distinguish one element from another. Forexample, first ranking criteria could be termed second ranking criteria,and, similarly, second ranking criteria could be termed first rankingcriteria, without departing from the scope of the present application.First ranking criteria and second ranking criteria are both rankingcriteria, but they are not the same ranking criteria.

The foregoing description, for purpose of explanation, has beendescribed with reference to specific implementations. However, theillustrative discussions above are not intended to be exhaustive or tolimit the application to the precise forms disclosed. Many modificationsand variations are possible in view of the above teachings. Theimplementations were chosen and described in order to best explainprinciples of the application and its practical applications, to therebyenable others skilled in the art to best utilize the application andvarious implementations with various modifications as are suited to theparticular use contemplated. Implementations include alternatives,modifications and equivalents that are within the spirit and scope ofthe appended claims. Numerous specific details are set forth in order toprovide a thorough understanding of the subject matter presented herein.But it will be apparent to one of ordinary skill in the art that thesubject matter may be practiced without these specific details. In otherinstances, well-known methods, procedures, components, and circuits havenot been described in detail so as not to unnecessarily obscure aspectsof the implementations.

What is claimed is:
 1. An off-grid power generating apparatus forpowering an electrical load, comprising: an engine; an alternatorincluding: a rotor coaxially coupled to the engine, the rotor includinga plurality of symmetric phase windings; a stator with a single phasewinding configured to generate an induced voltage, the single phasewinding being configured to be connected with the electrical load andthe phase windings of the rotor respectively and supply an outputvoltage to the electrical load and an excitation voltage to the phasewindings of the rotor; a voltage sensor configured to measure anamplitude of the output voltage; a current sensor configured to measurean amplitude of an alternating current applied to the electrical load;and a speed sensor configured to measure a rotation speed of the rotoror the engine; and an excitation control device operatively connected tothe engine and the alternator, wherein the excitation control device isconfigured to control the induced voltage generated in the single phasewinding of the stator by regulating the rotating magnetic fieldgenerated in the phase windings of the rotor; the excitation controldevice includes: a first calculating element configured to calculate aload power of the electrical load at least in accordance with themeasured amplitudes of the output voltage and the alternating current,and determine a desired rotation speed of the engine at least inaccordance with the calculated load power; a first modulating elementconfigured to modulate the speed of the engine at least in accordancewith the desired rotation speed of the engine; a second calculatingelement configured to determine a slip angle at least in accordance withthe rotation speed of the rotor or the engine measured by the speedsensor; and a third calculating element configured to determine a targetvoltage of the rotor at least in accordance with the amplitude of theoutput voltage measured by the voltage sensor; and a second modulatingelement configured to generate a modulating signal at least inaccordance with the target voltage of the rotor and the slip angle, andmodulate a frequency and an amplitude of an excitation current in thephase windings of the rotor with the modulating signal.
 2. The apparatusof claim 1, the alternator comprising a frequency conversion deviceconnected with the single phase winding of the stator, wherein thefrequency conversion device includes an inverter that provides theexcitation voltage to the phase windings of the rotor.
 3. The apparatusof claim 1, wherein the alternator includes a switch operativelyconnected with the electrical load and being movable from a firstposition to a second position by a user, the single phase winding of thestator includes a first segment and a second segment each of which hasat least one coil that are operatively and separately connected with theswitch, the first segment and the second segment are configured to be inseries connection at the first position of the switch to provide a highoutput voltage to the electrical load via the switch and in parallelconnection at the second position of the switch to provide a low outputvoltage to the electrical load via the switch, wherein: the voltagesensor is configured to measure the amplitudes of a first and a secondoutput voltage provided by the first and the second segment to theelectrical load; and the current sensor is configured to measure theamplitudes of a first and a second alternating current appliedrespectively by the first and the second segment to the electrical load.4. The apparatus of claim 1, wherein the alternator includes a switchoperatively connected with the electrical load and being movable from afirst position to a second position by a user, the single phase windingof the stator includes a first segment and a second segment each ofwhich has at least one coil that are operatively and separatelyconnected with the switch, the first segment and the second segment areconfigured to be in series connection at the first position of theswitch and in parallel at the second position of the switch to provide ahigh output voltage and a low output voltage respectively to theelectrical load via the switch.
 5. The apparatus of claim 1, wherein thethird calculating element is configured to determine the target voltageof the rotor with a closed control loop.
 6. The apparatus of claim 1,the alternator comprising an electrical circuit connecting the singlephase winding of the stator and the electrical load, wherein theelectrical circuit s configured in a way that the induced voltagegenerated in the single phase winding of the stator and the outputvoltage applied to the electrical load are at the same frequency.
 7. Acontrol method of an off-grid power generating apparatus for powering anelectrical load, the apparatus including an engine and an alternatorthat includes a stator with a single phase winding configured togenerate an induced voltage and a rotor with a plurality of symmetricphase windings, wherein the rotor is coaxially coupled to the engine,and the single phase winding of the stator is configured to be connectedto the electrical load and the phase windings of the rotor respectivelyand provide an output voltage to the electrical load and an excitationvoltage to the phase windings of the rotor, the method comprising:measuring an amplitude of the output voltage provided to the electricalload; measuring an amplitude of an alternating current provided to theelectrical load; measuring a rotation speed of the rotor or the engine;calculating a load power of the electrical load at least in accordancewith the measured amplitudes of the output voltage and the alternatingcurrent; determining a desired rotation speed of the engine at least inaccordance with the calculated load power; modulating the speed of theengine at least in accordance with the desired rotation speed of theengine; determining a slip angle at least in accordance with themeasured rotation speed of the rotor or the engine and the synchronousspeed of the alternator; determining a target voltage of the rotor atleast in accordance with the amplitude of the measured output voltage;generating a modulating signal at least in accordance with the targetvoltage of the rotor and the slip angle; and modulating a frequency andan amplitude of an excitation current in the phase windings of the rotorwith the modulating signal.
 8. The method of claim 7, the alternatorcomprising a frequency conversion device connected to the single phasewinding of the stator, wherein the frequency conversion device includesan inverter that provides the excitation voltage to the phase windingsof the rotor.
 9. The method of claim 7, wherein the single phase windingof the stator includes a first segment and a second segment, measuringan amplitude of the output voltage includes measuring the amplitudes ofa first and a second output voltage provided by the first and the secondsegment to the electrical load; measuring an amplitude of an alternatingcurrent includes measuring the amplitudes of a first and a secondalternating current applied respectively by the first and the secondsegment to the electrical load; calculating a load power of theelectrical load includes calculating a first and a second load power ofthe electrical load at least in accordance with the measured amplitudesof the first output voltage and the first alternating current, and thesecond output voltage and the second alternating current, and a totalload power by adding the first and the second load power, anddetermining a desired rotation speed of the engine at least inaccordance with the calculated load power includes determining thedesired rotation speed of the engine at least in accordance with thetotal load power; and determining a target voltage of the rotor includesdetermining the target voltage of the rotor at least in accordance withthe amplitude of the first or the second output voltage.
 10. The methodof claim 7, wherein the single phase winding of the stator includes afirst segment and a second segment, measuring the amplitude of theoutput voltage includes measuring the amplitudes of a first and a secondoutput voltage provided by the first segment and the second segment tothe electrical load, and a total output voltage when the first segmentand the second segment are in series connection; measuring the amplitudeof an alternating current includes measuring the amplitudes of a firstand a second alternating current applied respectively by the first andthe second segment to the electrical load; when the first segment andthe second segment are in series connection to provide the high outputvoltage, calculating a load power of the electrical load includescalculating a total load power at least in accordance with the measuredamplitudes of the total output voltage and either of the firstalternating current and second alternating current; determining adesired rotation speed of the engine includes determining the desiredrotation speed of the engine at least in accordance with the total loadpower; and determining a target voltage of the rotor includesdetermining the target voltage of the rotor at least in accordance withthe amplitude of the measured total output voltage.
 11. The method ofclaim 7, wherein the single phase winding of the stator includes a firstsegment and a second segment, measuring the amplitude of the outputvoltage includes measuring the amplitudes of a first and a second outputvoltage provided by the first segment and the second segment to theelectrical load, and a total output voltage when the first segment andthe second segment are in series connection; measuring the amplitude ofan alternating current includes measuring the amplitudes of a first anda second alternating current applied respectively by the first and thesecond segment to the electrical load; and when the first segment andthe second segment are in parallel connection at the second position ofthe switch to provide the low output voltage, calculating a load powerof the electrical load includes calculating a first and a second loadpower of the electrical load at least in accordance with the measuredamplitudes of the first output voltage, the first alternating current,the second output voltage and the second alternating current, and atotal load power by adding the first and the second load power;determining a desire rotation speed of the engine includes determiningthe desired rotation speed of the engine at least in accordance with thetotal load power; and determining a target voltage of the rotor includesdetermining the target voltage of the rotor at least in accordance withthe amplitude of the first or the second output voltage.
 12. The methodof claim 7, wherein the induced voltage generated in the single phasewinding of the stator and the output voltage applied to the electricalload are at the same frequency.
 13. The method of claim 7, wherein thealternator is set to operate at a speed equal to or less than asynchronous speed of the alternator.
 14. An off-grid portable generatorset for powering an electrical load, comprising: an engine; an inductionalternator including: a rotor coaxially coupled to the engine, the rotorincluding a plurality of symmetric phase windings; a stator with asingle phase winding configured to generate an induced voltage, thesingle phase winding being configured to be connected with theelectrical load and the phase windings of the rotor respectively andprovide an output voltage to the electrical load and an excitationvoltage to the phase windings of the rotor; a voltage sensor configuredto measure an amplitude of the output voltage; a current sensorconfigured to measure an amplitude of an alternating current applied bythe single phase winding to the electrical load; and a speed sensorconfigured to measure a rotation speed of the rotor or the engine; andan excitation control device operatively connected to the engine and thealternator, wherein the excitation control device is configured tocontrol the induced voltage generated in the single phase winding of thestator by regulating the rotating magnetic field generated in the phasewindings of the rotor; the excitation control device includes: a firstcalculating element configured to calculate a load power of theelectrical load at least in accordance with the measured amplitudes ofthe output voltage and the alternating current, and determine a desiredrotation speed of the engine at least in accordance with the calculatedload power; a first modulating element configured to modulate the speedof the engine at least in accordance with the desired rotation speed ofthe engine; a second calculating element configured to determine a slipangle at least in accordance with the rotation speed of the rotor or theengine measured by the speed sensor; a third calculating elementconfigured to determine a target voltage of the rotor at least inaccordance with the amplitude of the output voltage measured by thevoltage sensor; and a second modulating element configured to generate amodulating signal at least in accordance with the target voltage of therotor and the slip angle, and modulate a frequency and an amplitude ofan excitation current in the phase windings of the rotor with themodulating signal.
 15. The generator set of claim 14, the alternatorcomprising a frequency conversion device connected with the single phasewinding of the stator, wherein the frequency conversion device includesan inverter that provides the excitation voltage to the phase windingsof the rotor.
 16. The generator set of claim 14, wherein the alternatorincludes a switch operatively connected with the electrical load andbeing movable from a first position to a second position by a user, thesingle phase winding of the stator includes a first segment and a secondsegment each of which has at least one coil that are operatively andseparately connected with the switch, the first segment and the secondsegment are configured to be in series connection at the first positionof the switch and in parallel connection at the second position of theswitch to provide a high output voltage and a low output voltagerespectively to the electrical load via the switch.
 17. The generatorset of claim 14, wherein the single phase winding of the stator includesa first segment and a second segment, the voltage sensor is configuredto measure the amplitudes of a first and a second output voltageprovided by the first segment and the second segment to the electricalload, and a total output voltage when the first segment and the secondsegment are in series connection; and the current sensor is configuredto measure the amplitudes of a first and a second alternating currentapplied respectively by the first and the second segment to theelectrical load.
 18. The generator set of claim 17, wherein: the firstcalculating element is configured, when the first segment and the secondsegment are in series connection to provide the high output voltage, tocalculate a total load power at least in accordance with the measuredamplitudes of the total output voltage and either of the firstalternating current and second alternating current, and to determine thedesired rotation speed of the engine at least in accordance with thetotal load power; the first calculating element is also configured, whenthe first segment and the second segment are in parallel connection toprovide the low output voltage, to calculate a first and a second loadpower of the electrical load at least in accordance with the measuredamplitudes of the first output voltage and the first alternatingcurrent, and the second output voltage and the second alternatingcurrent, and a total load power by adding the first and the second loadpower, and to determine the desired rotation speed of the engine atleast in accordance with the total load power; and the third calculatingelement is configured, when the first segment and the second segment arein series connection to provide the high output voltage, to determine atarget voltage of the rotor at least in accordance with the amplitude ofthe total output voltage; and the third calculating element is alsoconfigured, when the first segment and the second segment are inparallel connection to provide the low output voltage, to determine atarget voltage of the rotor at least in accordance with the amplitude ofthe first or the second output voltage.
 19. The generator set of claim14, the alternator comprising an electrical circuit connecting thesingle phase winding of the stator and the electrical load, wherein theelectrical circuit s configured in a way that the induced voltagegenerated in the single phase winding of the stator and the outputvoltage applied to the electrical load are at the same frequency. 20.The generator set of claim 14, wherein the alternator is set to operateat a speed equal to or less than a synchronous speed of the alternator.